1. Field of the Invention
The present invention relates to electrostatic copiers.
More particularly, the invention relates to an electrostatic copier having a developing station which can be inserted into and removed from the copier as a unit.
2. The Prior Art
In electrostatic copiers an original is imaged onto the photosensitive surface of an endless travelling charge carrier to create a latent electrostatic image thereon. Toner particles are then attracted to the charged area of the electrostatic image; i.e. the latent image is developed to form a visible image. The toner particles forming this visible image are thereupon transferred to an image carrier, e.g. a sheet of copy paper, and fused to provide a permanent image on the same. Various ways of developing the latent image have been proposed, including cascade developing in which the toner material is cascaded over the photosensitive surface of the travelling charge carrier, and magnetic brush developing. For ease of servicing it is desirable to be able to insert the entire developing station into the machine as a unit, and to remove it in the same manner and such as an arrangement has been proposed in e.g. German Published Application (DT-OS) 1,961,836.
The quality of the developing process--and hence the quality of the finished copy--depend to a very large extent upon a precise maintenance of the position of the developing unit or station relative to the charge carrier surface; this is important in almost all known developing processes, but especially so in cascade developing and magnetic brush developing. To assure such maintenance in constructions in which the developing station is removable as a unit, it has been proposed to use space rollers which engage the charge carrier surface to maintain the desired relative position of the station to this surface. However, the charge carrier surface is highly susceptible to damage and the use of such rollers inherently involves the danger of such damage, which may occur e.g. when the charge carrier is removed for inspection or replacement. To avoid this, the prior art proposes to retract the developing station--together with associated components--out of contact with the charge carrier surface before the charge carrier is removed. This, however, requires a rather complicated construction which is correspondingly expensive.
A somewhat different approach is adopted in German Published Application (DT-OS) 2,436,301 where the removal of the charge carrier is facilitated by combining it with a surface-cleaning device to form a removable unit therewith. This, however, does not change the fact that in this construction, also, the spacing between the developing station and the charge carrier surface is maintained by space rollers which must engage the charge carrier surface.